gherardi



B. GHERARDI.

SYSTEM FOR THE EQUALIZATION 0F TRANSMISSION LINES.

APPLICATION FILED 050.21, I9I7.

1 ,396,930, Patented Nov. 15, 1921.

5 SHEET SHEET l.- 3.;

INVENTOR;

A TTORNEY B. GHERARDI.

SYSTEM FOR THE EQUALIZATION 0F TRANSMISSION LINES.

APPLICATION FILED DEC. 27, 1917.

1,396,930, Patented Nov. 15,1921.

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B. Ghemrdz' B. GHERARDI.

SYSTEM FOR THE EQUALIZATION 0F TRANSMISSION LINES.

APPLICATION FILED DEC. 27. 1911.

96,930. Patented Nov. 15,1921

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B. Ghana db ATTORNEY B. GHERARDI.

SYSTEM FOR THE EQUALIZATION 0F TRANSMISSION LINES.

APPLICATION fILED DEC.27,19I7-.

Patented Nov. 15, 1921.

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B. Ghenmiz 42 5 w98765452404234 w hag wwmmwsgk w Q 0 0 0 0 .nm 0 0 0 0 0A TTORNEY UNITED STATES PATENT OFFICE.

BANCROFT GHERARDI, OF BAYHEAD, NEW JERSEY, ASSIGNOR TO AMERICAN TELE-PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

SYSTEM FOR THE EQUALIZATION OF TRANSMISSION-LINES.

Application filed December 27, 1917.

To all whom it may concern:

Be it known that I, BANCROFT Gnnmnoi, residing at Bayhead, in the countyof Ocean and State of New Jersey, have invented certain Improvements inSystems for the Equalization of T ransmission-Lines, of which thefollowing is a specification.

This invention relates to telephone systems and more particularly to theprovision of means whereby the electrical characteristics of telephonelines may be equalized or rendered more uniform.

Telephone lines have, in general, different electrical characteristicsvarying with the lengths of the lines. For any particular type of lineconstruction which may be employed the resistance, capacity, inductanceand leakage of the circuit increase with the length. These variations inthe electrical -haracteristics of the different subscribers lines in atelephone system introduce a number of variable effects in the telephoneservice rendered from differentstations. F or example the differentelectrical characteristics result in different degrees of attenuation inthe alternating telephone current passing over the subscribers lines andthus give louder and more effective telephonic communication tosubscribers connected by means of short lines than to subscribersconnected by means of long lines. As a result, a subscriber having along line, when connected with any other subscriber, does not receive asgood telephone transmission as would a subscriber having a short linesimilarly connected. Furthermore, in the common battery systems now sogenerally employed in all but small communities, the resistance of theline causes a further effect on the transmission in that it reduces theamountof direct current which the transmitter receives from the centralollice battery and so renders it less eflicient in the generation of thealternating telephone current. Likewise in common battery systemsemploying electro-inagnetic receivers re. e. receivers not including apermanent magnet but depending for their energization upon the supplycurrent,this variation in current supply introduces changes in the011iciency of the receiver. A further loss in transmission occurs if theimpedance of the subscribers line differs from the impedanceSpecification of Letters Patent.

Patented Nov. 15, 1921. Serial No. 209,116.

of the terminal or circuit to which it is connected, and therefore thisloss, which is known as a reflection loss, varies as the impedance ofthe line varies.

here an anti-side tone type of substation circuit is associated with thetransmission line, it is necessary to balance said line, of which thesubscribers line is an important part, by an artificial line.Difficulties in obtaining a uniform anti side tone arrangement areincreased by the variations in the electrical characteristics of thesubscribers lines. These variations also affect the design of trunks andtoll lines. lVhen two subscribers located in different central officesare connected to each other by means of trunks or toll lines, it hasbeen necessary to design such trunks or toll lines so as to obtain theefliciency necessary to give satisfactory service to the subscribershaving the longest subscribers lines, thus requiring greatly increasedexpenditure over and above what would have been needed had thetransmission efliciency of the long subscribers lines been equal to thatof the short ones.

Attempts have been made heretofore to overcome certain of thesedifficulties by reducing to some extent the direct current supply forshort or low resistance subscribers lines. These methods were defectivenot only because they did not render the surrent supply constant forlines of different resistances but for the further reason that noattempt was made to equalize the electrical characteristics foralternating telephone current thereby failing to consider the otherserious difficulties mentioned above. The present invention has for itsobject the substantial elimination of the difiiculties above mentioned,together with the accomplishment of other ends more fully appearinghereafter. These objects are attained in accordance with the present.invention by the provision of means whereby within any desired limitssubstantially uniform electrical characteristics may be securedregardless of the length of the subscribers lines.

lVhile the means for obtaining these uniform electrical characteristicsand thus obtaining uniform transmission may within the scope of thisinvention assume a wide range of forms, it has been found thatsatisfactory results may be secured by connectnations of series andshunt impedance elements so chosen as to simulate the electricalconstants of different lengths of the subscribers line can be used tomake all the subscribers lines have practically the same electricalcharacteristics. For the type of subscribers line in general use theleakage and inductance are small and a very close approximation of allthe characteristics oi a length of line can be obtained with acombination of series resistances and shunt capacities. It has beenfound in many cases that satisfactory results in the equalization oftransmission can be secured by merely inserting at some point in thesubscribers line series impedance elements whose values de pend upon thelength and electrical characteristics of the line. Certain of the aboveresults may also be secured by using shunt impedance elements. They mayalso be secured by varying the. design and characteristics of theinduction coil forming a part of the subscribers telephone set.

One of the great advantages of the system described above issimplification in the conditions under which the transmitter and theelectromagnetic receiver must be operated. Since the exciting current ofthe transmitter is kept practically uniform independent of the length ofthe line on which the transmitter is used, it will be possible to designthe transmitter for use with reference to this current, and since thiscurrent will be low well as uniform the transmitter may be designed togive a much greater efiiciency than would be possible with the samecurrent if the transmitter were required to operate with wide variationsin current values. Similarly, the conditions are simplified forelectromagnetic receivers. As a result not only will the transmissionover lines of different lengths be rendered substantially uniform butthe average transmission will be better than that now obtained. In fact,it is possible, by the proper design oi the transmitter and receiver, orby increasing the battery voltage at the central oftice. or by otherwell known means. when employed in connection with the equalization of?the lines as above pointed out, to obtain by this invention oversubscribers lines of any reasonable length transmission which is as goodor better than that now obtained on very short lines. The equalizationof transmission in accordance with this invention does not involvereducing the transmission over short lines to that secured over thelongest line, since it is possible by Well known means to increase thetransmission to whatever extent may be desired.

The elimination of the variation in efficiency of subscribers lines andthe substation equipment associated therewith renders possible asubstantial increase in the transmission efficiency of the telephonesystem. The telephone switchboards and telephone lines, particularly thetrunk lines and toll lines, cannot have impressed upon them energybeyond a certain amount without bringing in difiiculties from crosstalk.Similarly the increase in sensitiveness of the receiver is limited bythe crosstalk and noises caused by extraneous electrical sources knownas inductive disturbances which could be made audible by a verysensitive receiver. By the use of equalized subscribers lines, it ispossible to work the switchboards, trunks and toil lines up to thepermissible limit on all connections, whereas without this invention thelimit would be fixed by the difliculties encountered in connection withthe shorter subscribers lines.

Furthermore, at the present time all trans- .iitters and receivers mustbe designed with due regard to the connecting together of two shortsubscribers lines without intermediate trunk or toll line. Theseinstruments cannot be made more powerful and more sensitive than wouldbe practical for use under such conditions, thus determining the limitfor short lines and placing the longer lines substantially below thatlimit. This difliculty is overcome by means oi the arrangements of thisinvention.

Another advantage may be cited for the forms of this invention thatemploy series impedanoes. In common battery systems the current goingout on the subscribers line is employed to operate relays at the centraloffice for arious purposes. The relays employed must be designed so thatthey will operate satisfactorily at one minute on. a short line taking agreat deal of current and at the next minute on a long line having; onlya very small current. This imposes serious limitations on the design ofthe relays and increases the difliculty in maintaining them insatisfactory operating adjustment. When the lines are equalized inaccordance with this invention these difficulties are materiallyreduced.

The invention will now be clear from the following description when readin con.- nection with the accompanying drawings, in which Figure 1 is asimplified diagram of a transmission circuit involving two subscribersin the same central ofiice and employing the arrangements of theinvention; Figs. 2-11 inclusive are diagrams of substation circuitsillustrating different forms of the invention; Fig. 12 is a diagram of atesting circuit whereby the transmission equivalent of the variouscircuits may be determined; and Figs. 1318 inclusive are curvesillustratingvarious features of the invention.

Referrin to Fig. l, a transmission circuit is shown in simplified formconnecting substations S and S through a central station S A battery Bis provided at the central station from which direct current is suppliedover the lines L and L to the sub stations at S and S respectively. Thesubstation arrangement at S comprises the usual transmitter T andpermanent magnet receiver R the latter being arranged in a local circuitinductively related to the line circuit through an induction coil I inthe usual manner. The substation circuit at station S comprises atransmitter T and an electro-magnetic receiver R; the two elements beingfor the purpose or illustration arranged in a simple series circuit inorder that the receiver may be energized by the direct current flowingover the line L, although it will be understood that any substationarrangement may be employed in which the direct current flows throughthe receiver. The two different forms of substations are shown in Fig. 1for the purpose of illustrating two different forms of the invention, itbeing understood that the line may terminate in any desired type ofsubstation circuit. The lines leading to various substations inaccordance with the present practice vary in length and resistance sothat the current supply flowing over the lines from a common centralenergy source such as B will vary with the electrical characteristics ofthe line, a greater amount of current flowing over the lines of lowresistance. Furthermore the amplitude of the alternating talkingcurrents will vary with the length and resistance of the line aside fromthe variations in current supply. This results in a correspondingvariation in the telephone transmission over the lines.

These factors as well as other considerations already discussed render.it desirable that the lines be equalized with regard to their electricalcharacteristics. This result may be accomplished in various ways and asindicated in Fig. 1 the line L is equalized by inserting in the line animpedance element comprising series resistances P and P and shuntcapacity C. The network thus formed is substantially equivalent to adefinite length of line of the type of line L so that the line L and thenetwork together constitutes a conducting system whose electricalcharacteristics are substantially equivalent to the characteristics of alonger line whose length is the standard for equalization. The networkmay be inserted in the line at any point but as shown is inserted at thecentral station S A sufficient approximation of the desired result mayin some cases be attained by a simpler form of impedance element. Thusthe line L is shown as being equalized by a simple series impedance Pinserted in the line at the substation S and which may be of any desiredcharacter but preferably is an element, either inductive ormnrinductive, whose principal characteristic is its resistance. lVhileconventionally shown as a separate element it is obvious that theimpedance may be incorporated in existing elements of the circuit. Thusinstead of a separate impedance l the winding 01" the induction coil 1may be so wound as to proride the necessary impedance. Switch hooks Eland H may be provided at the stations 9 and S in order to control thecircuits ot the impedances and substation a )paratus.

propery proportioning these impedtil. -c clenients as hereinafter morefully explained, the transmission over the lines L and L will besubstantially equal regardless of their normal resistance. lV here aseries impedance or resistance element is employed, as in Fig. 1, theinserted impedance will be greatest for short lines and will begradually decreased with increasing length of line until for lines or agiven length no impedance whatever will be inserted. By this arrangementnot only will the attenuation of the voice currents be substantially thesame for all lines which are equalized, but the direct current suppliedto the substation will be substantially the same for all equalized linesand the transniitters in substation ('ll'Cllll'S ot' the type shown at Sand 5 will in all cases receive substantially the saline cner; 'zingcurrent. As this current will be no 3 eater than that recciv'd thelonget line equalized it is possil design the transmitter so as to havea g eliiciency for small currents. since there is no danger that thetransmitter will be subjected to large currents which will cause it toburn. Consequently the insertion of impedance elements in short line:does not necessarily result in reducing the transmission over all linesto that obtained over the longest lines as it is possible by increasingthe elliciency of the transmitter to increase the transmission obtainedover all lines above that heretofore obtained over short lines. Theuniform current supply obtained also simplifies the conditions for theuse of an electromagnetic receiver as shown at sta tion S. One of theditliculties in connection with the use of this type of receiver hasbeen that its eiliciency varies with the electrical characteristics ofthe line, a ditliculty which no longer obtains in accordance with thepresent invention. In addition to the use of a more etiicienttransmitter and receiver, it is possible to increase the uniformtransmission obtainable in accordance with the equalization methodsabove described,

by increasing the potential of the battery at thecentral station, or byother well known means, so that within reasonable limits any desiredtransmission may be obtained over any line.

Satisfactory results may in some case: he obtained by shunting theimpedance ment across the line as shown in it and 3. By locating theimpedance shunt at the substation, the direct current supply flowingover lines of different lengths varies some what, but the currentsflowing through the substations proper may be 1112 equal. l( gardless ofthe character the lines it the inmedance shunts be properlyproportioned. The iunedance employed may be either inductive ornon-inductive as desired. The impedance shunt l may be connected acrossthe terminals of a substation employingeither a permanent magnet re. oran electromagnetic receiver, the Former arrangement being illustrated in2 and the latter in Fig. 3.

It not necessary in all cases to shunt the entire substation, it beingsufficient to provide a shunt about some element thereof. In Fig. 4- isshown a substation circuit employing a permanent magnet receiver with animpedance element 1 in shunt of the transmitter T. By properlyproportioning the impedance P in accordance with the resistance of theline, the transmitter currents for all lines may be made equalregardless of the current actually flowing in the lines, although thereceiving cfliciency may vary somewhat. F ig. 5 shows the samearrangement applied to the transmitter of a substa tion circuitemploying an electromagnetic receiver.

A somewhat similar result may be obtained by shunting the receiver Rinstead of the transmitter as shown in Figs. 6 and 7. In F lg. 6 theshunt is connected across the terminals of an electromagnetic receiverR. while in Fig. 7 it is connected acros the terminals of a permanentmagnet receiver. In stead of shunting the transmitter or receiver, it ispossible in some types of substation circuit to include an impedance inseries with some element of the sub tation such as the transmitter orreceiver. Thus in Fig. 8 for example an impedance 1 is included inseries with the transmitter T for the purpose of equalizing thetransmission.

While in general there is greater reason for equalizing the transmissionover lines sup plied with current from a common central energy sourcethan would be the case with lines employing'local battery sets, even inthe latter case it may be desirable to equal: ize the transmission. Anarrangement for producing this result is shown in F ig. 9, in which animpedance element P, either inductive or non-inductive, is included inthe local circuit comprising the "*itery B and the transmitter T. Bysuitably proportioning; the impedance element 1 the energy transmittedover lines of different lengths may be made the anie. Instead ofinserting the impedance in the local circuit it may be included in theline circuit as shown in 10. In either case the impedance element may beincorporated in the induction coil. A. similar effect may be obtained byeliminatino' one or more cells of the local battery as indicated in Fig.11.

In order to understand more fully how the transmission varies with linesof different electrical characteristics and how to proportion theimpedance elements in order to equalize transmission, attention iscalled to the curves shown in Figs. 13 to 18 inclusive. Referring moreparticularly to Fig. 13, the curve A is a transmission curve indicatingthe variation in transmission of a Well known type of substation setwith lines of diiferent resistance, the loop resistances being plottedas abscissae and the transmis sion efficiency being plotted asordinates.

The manner in which a curve such as A is obtained may be understood byreference to Fig. 12 in which two substation circuits, X and Y, areconnected to a zero loop, including one Winding of a transformer coiland a battery whose potential is equal to that of the usual centralenergy source. The substation X, whose transmission is to be measured,is first directly connected to the primary of a transformer 23. Thetransformer 23 is associated with the transformer 2st of the circuit Ythrough a variable length of cable, either real or artificial. As shownan artificial cable N is used, said cable bein preferably made up ofsections such as 10, 11, 12, 13, 14;, 15, etc., each section compr ngseries impedance and shunt capacity of such values that a section isequiv alent to a unit length of standard cable. The number of sectionsemployed in the artificial cable N should be sufficient to produce acable which Will still be electrically long' when the greatest number ofsections is cut out in making the tests hereinafter described. Thesecondary of the transformer 21 may be adjustably connected to theartificial cable N so that the number of sections included may bevaried.

Assuming now that a transmission test is to be made of transmitter T ofthe sub station circuit X, the substation X is first directly connectedover the switch 22 to the transformer 23, and any desired number of hesections of the artificial cable N are in cluded in the circuit. Thetransmitter T, is actuated by a sound and the resultant sound producedby the receiver R, of the station Y is observed. The sound thus obtainedis then considered the measure of the transmission obtained when thesubstation X is connected to a zero loop. The substation X is thenconnected with the transformer 2 through a section of real cable of thetype with which the substation is to be used, or through one section,say 16, of an artificial cable, said section being made to simulate aunit length of the real cable. The transmitter T is again actuated by asound of the same volume as before and the observer at Y varies theconnection of the transformer 2i to the standard artificial cable Ncutting out a suflicient number of sections so that the same volume ofsound is heard in the receiver R, as be fore. -When the same volume ofsound is obtained, the transmission loss due to the inclusion of oneunit, 16, of the cable to be tested in circuit wit-h the substation X,is equal to the loss which would have been obtained by introducing thenumber of sections cut out of the artificial cable N instead. In otherwords the introduction of a unit of the cable N produces a trans--mission loss equal to the introduction of a certain number of units ofthe standard cable l Succeeding tests are now made by successivelyincluding in circuit with the substation X additional units of the cableN and noting the number of sections of the cable N which must beeliminated in order to get the same volume of sound in the receiver R,in each case. If then the resistances of the number of units of thecable N be plotted as abscissze and the cor responding number of unitsof the cable N be plotted as ordinates, curve A will result. This curvemay be taken as the curve of a standard transmitter T when connected instandard substation circuit with loops of different length.

Suppose no it is desired to obtain the transmission curve of atransmitter T of a substation circuit Z, said transmitter T being ofgreater efiiciency than the transmitter T The substation X may now bedisconnected and the substation Z connected to the transformer 23 bymeans of the switch 22. The variable impedance element P should be soset that no impedance is in series with tl e substation and theimpedance of the substation Z should be equal to that of the substationX. It the transmitter T be now actuated by a sound as before, theobserver at station Y mav add a sufficient number of units to theartificial cable N so that the same sound. is heard in the receiver R,as under the zero loop condition when the transmitter T, was tested. Thenumber of units added to the artificial. cable N now represents thenumber of transmission units of gain produced by the transmitter T underthe zero loop condition. Assuming that the gain is equal to five unitsof the cable N, the first point of the curve B, which is a transmissioncurve of the transmitter T, is obtained. A similar series of tests isnow made by adding successive units of the cable N and noting theequivalent number of units of the standard cable N which must beeliminated. .The curve B now results, and it will be noted that thiscurve represents a gain of substantially five transmission units overthe curve A for all line resistances. The curve 1) represents thevariation of the battery supply current with different lengths of linefor both the transmitter T and the transmitter T the units of currentbeing plotteu as ordinates.

If now it is desired to make the transmis sion with the transmitter Tconstant over a given range such that the, transmission obtained overthis range is equal to the zero loop condition of the transmitter T thesubstation Z is again connected directly to the transformer 23 and asufiicient amount of resistance P is included in the circuit, so thatwith the original amount of artificial cable N in circuit the same soundis produced in the receiver R, as with the transmitter T. under the zeroloop condition. Without varying the artificial cable N successive unitsof the cable N, are connected in circuit and the resistance P reduced,so that the same sound is heard in the receiver R, and this is continueduntil the resistance P is reduced to zero. The curve F may now beplotted to indicate the number of units of resistance necessary tomaintain the transmission .constant with different lengths of the cableN,. In obtaining this curve resistance units are plotted as ordinates.The heavy line curve C is now a straight line from zero up to the pointof its intersection with the curveB, indicating that the transmissionobtained up to this point is equal to the transmission obtained with thetransmitter T, under the zero loop condition. From this point the curveC will be the same as the curve B as there is nothing to compensate forfurther increase in the loop resistance. The heavy dotted line curve Erepresents a corresponding battery supply curve, said curve beinguniformover the same range as the transmission i uniform.

From the curves shown in Fig. 13 it is apparent that transmission may beequalized over lines up to almost three units of loop resistance and atransmission still be obtained equal to the best transmission obtainablewith the standard transmitter T,. If it is desired to equalize thetransmission over a greater range, the initial impedance P for the .zeroloop condition, may be increased. thereby decreasing the transmittingefficiency over the equalized range thus bringing the curve Cv up fromzero a certain number of transmission units. The curve C may then againbe continued in a horizontal direction as the series impedancedecreases, until the curve C crosses the curve B.

Fig. 14 represents the receiving efliciency obtained by the use of anelectromagnetic receiver of a well known type. The curve D as beforerepresents the variation of battery supply current with different lineresistances. A test is first made with the substations X and Y connectedto the transformers 23 and 24 by zero loops, the transformers beingconnected through a suitable number of units of standard artificialcable N. The transmitter T may be actuated by a sound and thecorresponding sound made in the standard receiver noted. This may now beconsidered a measure of the receiving efficiency with the receiver onzero loop. If now the substation X be disconnected and the substation Zincluding the electromagnetic receiver R be substituted. the impedance Pbeing eliminated, the transmitter T. may again be actuated and asufficient number of units of the standard cable N be added to thecircuit to produce the same volume of sound in the receiver R asobtained before with receiver R... The number of units of standard cableN added in order to produce this result, represents the gain in transmission due to the superior efiieiency of the receiver R as compared withreceiver R- under the zero loop condition. This gives the first point ofthe curve G which it will be noted represents a gain of about 1.6 units.Similar tests may be made, successively including additional units ofthe cable N.. By plotting the variations in the receiving efiiciencywith respect to variations in the line resistance, we obtain the curveG. Comparing this curve with the curve D which represents the batterysupply current, it will be seen that the receiving eiiiciency isgreatest when the battery supply current is a little over .06.corresponding to a loop of about 2.8 units. and that the receivingeiliciency is reduced when the length of circuit is greater or less thanthis value of 2.8 units. If then the battery supply current bemaintained constant at this value. the receiving eificiencywillbeconstant. Accordingly with the substation Z connected directly tothe transformer 23 and the same amount of artificial cable connected asin the initial test. the impedance element P may be adjusted so as toinclude suflicient impedance in the circuit to bring the battery supplycurrent to the desired value. As units of the cable N are connected incircuit. the impedance P may be decreased as shown by the curve F andthe batterysupply current will remain constant as shown by the curve E.By. observing the sound in the receiver B it will be noted that thevolume of sound obtained is constant and equal to'a gain of about threeand onehalf units of the standard cable- This condition obtains until aloop of 2.8 units is reached when the compensating resistance reduced tozero and from this point on the battery supply current falls off asshown by the curve E and the receiving eiiiciency decreases as shown bythe curve H. Comparing curve H of Fig. M and curve C of Fig. 13 it willbe seen that both the transmitting elliciency and receiving efiiciencyare constant with loops up to about 2.8 units, and over this range thetransmission e'fiiciency of lines of different lengths will be the same.It will in general be possible to increase the range over which thetransmission is equalized, by inserting a greater resistance for thezero loop condition and accordingly reducing the resistance withincreasing length of line. This will of course lower the horizontal partof the curve E, thereby elevating the horizontal portions of the curvesC and H, so that the resultant transmission will be less but thehorizontal portions of the curve will be longer before intersecting withthe curves B and G respectively.

The method above described for obtaining the curves of Figs. 13 and 14are sutficiently accurate for practical conditions where a simpleresistance element is employed for equalizing the lines. here greaterrcfinenient is necessary and it is desired to equalize the lines as toall their electrical characteristics. the more complicated compensatingnet-works thereby necessitated may be determined by well known methods.

Fig. 15 shows the curve for the transmittcr T when shunted by acompensating impedance element as shown in Fig. Fig. showing thecorresponding curves for the receiver R of said figure, the receiver Rbeing an electromagnetic receiver such as that used in obtaining thecurve of Fig. 14 and the transmitter T being a transmitter of increasedefliciency such as that used in obtaining the curve of Fig. 13.

In Fig. 15 the curves A, B and D correspond to the same curves in Fig.13. The curve F indicates the value of shunt impedancc necessary tomaintain the transmitting efiiciency constant so as to obtain ahorizontal curve C for all loop resistances from zero up to about 2.8units. The heavy dotted line curve the battery supply current curveobtained when the transmitter is shunted by an impedance element. Acomparison of curves D and E indicates that in general a greater currentflows over the line when the transmitter is shunted by a low impedancethan the case when the transmitter is unshunted. the two curvesapproaching each other as the shunt resistance is increased. lVhen aloop resistance of about 2.8 units is reached. the two curves merge.since from this point on no compensating element is provided.

Referring to Fig. 16 which. shows the re lOO ceiving curves, the curvesD, E and F are the same as the corresponding curves of Fig. 15, and thecurve G corresponds to the same curve of Fig. 14. The curve H may beobtaincd by tests similar to those described in connection with thecurves of Figs. 13 and 1%, said curve representing the efliciency withthe receiver R of the circuit in Fig. 5 under the same condition ofbattery supply and shunt resistance as in Fig. 15. It will be noted thatunder the zero loop condition the receiving efficiency is less when thetrans mitten is shunted than under the condition when no shuntis used.This of course follows from the fact that the shunt reduces thetotal'resistance of the substation. thereby increasing the batterysupply current flowing over the line and through the receiver asindicated by the curve E. Since the receiving efficiency is greater withthe current supply obtained over a loop of about 2.8 units, the curve IIwill in general be higher than the curve G until a line resistance of2.8 units is reached, when the two curves merge and continue before. Acompari son of the curves C and H of Figs. 15 and 16 respectively, showsthat over the compen sated range the transmitting efiiciency remainsconstant while the receiving efficiency increases with increase in thelength of loop. In the curves heretofore discussed, it has been assumedthat the values of the series or shunt impedances, as the case may be,vary directly with the line resistance. In practice, however, thiscondition would not obtain but definite impedance elements would beprovided for lines approximating certain definite lengths. Consequentlythe vari ation of the impedance element with the line resistance wouldnot be uniform, but would be l y definite steps. This condition is indicated by the curves of Fig. 17 in which F indicates the variation ofthe series resist ance with the line resistance, E the corre spondingcurrent supply curve, C the transmitting curve and G the receivingcurve. It will be noted that the curves C and G as well as the batterysupply curve E, are not straight lines as in Figs. 13 to 16 inclusive,but are jagged or irregular. The results shown by the curves, however,constitute a sufficient approximation of the desired con dition tosatisfy practical requirements.

The curves of Fig. 18 indicate how it is possible to increase the rangeover which equalization takes place by increasing the potential of thesource of current supply, at the same time increasing the compensatingimpedance so that the actual current supply remains the same as before.In Fig. 17, under the zero loop condition, a series impedance of about235 units was employed in order to obtain the desired battery supplycurrent. In Fig.18 the series impedance employed under the zero loopcondition-is about 3'70 units. The potential of the battery is thenincreased by an amount sufiicient to bring the battery supply current upto the same initial value as that obtained in Fig. 17. By this means theequalization of the transmission may be extended to a line having aresistance of about 4.4 units before the compensating impedance becomeszero.

From the curves above given by way of illustration it will be apparenthow the impedance elements of the various circuit arrangements discussedmay be proportioned in order to equalize the transmission over anydesired range. Furthermore, it will be apparent that by the provision oftransmitters or receivers of increased efficiency, or by increase of thebattery potential or by other well known means it is possible to bringthe transmission to any desired value over the compensated range.

By means of the invention hereinbeior-e disclosed the service isimproved by provid ing uniform transmission of better grade than theaverage heretofore obtainable at the same time reducing crosstalktroubles and operating the transmitter and receiver under uniformconditions. In addition to the improvement in service obtained, largeeconomies result due to the saving in battery supply current and thedecrease in maintenance expense. Furthermore, since toll lines and trunklines must be designed so that proper transmission is obtainable oversuch lines when connected to subscribers lines of the lowest eflicioncy,great economies may now be secured in the design of trunk and tolllines, since they may be designed with rcfcrence to transmissionconditions obtaining when connected to uniform subscribers circuits ofgreater efiiciency than the low grade circuits which had to beconsidered heretofore.

It will of course be understood that while in this specification it hasbeen convenient to consider the properties of different lines in termsof the length or resistance of the line. these terms are all to beconstrued in the broadest sense, it being evident that other factorsthan length and resistance may be taken into consideration in equalizingthe lines. It will also be obvious that the general principles hereindisclosed may be embodied in many other organizations widely differingfrom those illustrated and described, without departing from the spiritof the invention as defined in the following claims.

Vhat is claimed is:

1. In a. telephone system, a central station. a plurality of outlyingstations, lines extending from the central station to each outlyingstation, each line having electrical characteristics which differ withrespect to direct currents and telephone currents. sa d characteristicsbeing different for diiferent lines, a substation set at each outlyiistation including a transn'iitter and a receiver, a common source ofcurrent supply for said sets at said central station, and means todetermine that the direct currents supplied to said lines will besubstantially the same and the voice currents impressed on the receivers will be substantially equal.

2. In a telephone system, a central s a tion, a plurality of outlyingstations, lines extending from said central station to eachv outlyingstation, each line having a different resistance for d rect currents andtor telephone currents and the resistances of different lines differingfrom each other, a substation set including a telephone at each outlyingstation, and an impedan e element tor equalizing the transmission ofvoice currents delivered to said telephone, the impedance of saidelement being predetermined in accordance with the resistance ot theline so that the transmission between any two outlying stationsinterconnected through said central station will be uniform.

In a telephone system, a ventral station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having electrical characteristicswhich are different for direct currents and for tele phone currents andthe electrical characteristics of the various lines differing from eachother, a substation set including a transmitter on each line at theoutlying station, and an impedance element for equalizing thetransmission of energy generated by said transmitter, the impedance ofsaid element being predetermined in accordance with the electricalcharacteristics of. the line so that the transmission between substationsets of any pair of lines interconnected through said central stationwill be uniform.

4:. In a telephone system, a central station, a plurality of outlyingstations. lines of differing lengths extending from said central stationto each of said outlying stations, :1 substation set including atransmitter and a receiver on each line at the outlying station, acommon source of current supply for said transmitters at the centralstation and means to equalize the transmission of energy to thereceivers over the different lines, said means also preventing anexcessive current from flowing through the transmitters on relativelyshort lines.

5, In a telephone system, a central station, a plurality of outlyingstations, lines extending to said outlying stations, each line havingresistances which are different with respect to direct currents andtelephone cur-- rents, and the resistances of the various linesdifi'ering from each other, substation for each line at the outlyingstations and means having electrical values predetermined in accordancewith said resistances to equalize the transmission of voice currentsover any pair of lines interconnected through said central station.

6. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each oi saidoutlying stations, said lines havinc, resistances which differ withrespect to direct currents and telephone currents and the resistances ofthe various lines differing from each other, substation sets for saidlines at said outlying stations, a common source of current supply forsaid lines at said central station, and means to equalize the voice currents received by said substation sets regardless of the resistances ofthe lines.

'7. In a telephone system, a central station, a, plurality ot' outlyingstations, lines extending from said central station to each of saidoutlying stations, said lines having electrical characteristics whichdiffer with respect to direct currents and telephone currents, saidelectrical characteristics of the various lines differing from eachother, substation sets for each line at said outlying stations and fixedimpedance elements for said lines so proportioned with respect to theelectrical characteristics of the lines and so related thereto that theline current flowing through said substations will be substantially thesame for any pair of lines interconnected through said central station.

8. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, said lines having electrical characteristics whichdiffer with respect to direct currents and alternating currents and saidelectrical characteristics of the various lines differing from eachother, substation sets for each line at each outlying station, eachsubstation set including a tele phone instrument, a fixed impedanceelement so proportioned with respect to the electrical characteristicsof the lines and so related thereto that the line current flowingthrough said telephone instruments will be substantially the same forany pair of lin interconnected through said central station 9. In atelephone system, a central station a plurality of outlying stations,lines extending from said central station toeach of said outlyingstations, each of said lines having electrical characteristics whichdiffer with respect to direct currents and telephone currents and theelectrical characteristics of the various lines differing from eachother, substation sets for said lines at each outlying station and fixedimpedance elements serially included in the lines and so proportionedwith. reference to the electrical characteristics thereof that the linecurrent flowing through different substations will be substantially thesame for any pair of lines interconnected through'said central station.

10. In a telephone system, a central station, a plurality of outlyingstat-ions, lines extending from said central station to each of saidoutlying stations, said lines each having resistances which differ withrespectto direct currents and telephone currents and the resistances ofthe various lines differing from each other, substation sets for eachline at said outlying stations and fixed impedance elements seriallyincluded in the lines at the substations and so proportioned withreference to the resistances thereof that the line current flowingthrough different substations will be substantially the same for anypair of lines interconnected through said central station.

11. In a telephone system, a central station, a plurality of outlyingstations, lines extending from Said central station to each of saidoutlying stations, each line having electrical characteristics whichdiffer with respect to direct currents and telephone currents and theelectrical characteristics of the various lines differing from eachother, substation apparatus for each line including a transmitter andreceiver at the outlying sta tion, and means for equalizing the energytransmitted from a transmitter over any pair of lines interconnectedthrough said central station and for equalizing the energy received overany pair of said lines by the receivers.

12. In a telephone system, a central station, a plurality of outlyingstations, telephones at said outlying stations, lines of difieringlengths extending from said central station to each of said outlyingstations, a common source of battery supply current at said centralstation for energizing said telephones, and means to prevent a greatertransmission of voice currents over short lines than over long lines.

13. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each line having electrical characteristics whichdiffer with regard to direct currents and telephone currents and theelectrical characteristics of the various lines differing from eachother, a substation set including a transmitter and a receiver on eachsubscribers line at the outlying stations, and means for so control lingthe generation of voice currents by the transmitter of any line and thetransmission thereof that substantially the same energy will be receivedby the receiver of any other line to which it is interconnected throughsaid central station.

14. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each outlyingstation, each of said lines having electrical characteristics whichdiffer with respect to direct currents and alternating currents and theelectrical characteristics of the various lines differing from eachother, a substation set in luding a transmitter and a receiver on eachline at the outlying station, and fixed impedance elements soproportioned and constructed with reference to the electricalcharacteristics of the individual lines that substantially thesame'amount of energy will be transmitted from the transmitter of oneline to the receiver of any other line with which it is connectedthrough said central station. a

15. In a telephone system, a' central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having resistances which differwith respect to direct currents and telephone currents and theresistances of the various lines differing from each other, asubscribers set including a transmitter and a receiver on each line atthe outlying station thereof, and means having electrical valuespredetermined in accordance with said resistances for rendering thetransmission of telephone currents uniform over any pair of linesinterconnected through said central station.

16. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having electrical characteristicswhich differ with respect to direct currents and telephone currents andthe electrical characteristics of the various lines differing from eachother, and means predetermined in accordance with said characteristicsfor rendering said electrical characteristics uniform both as to directcurrent supply and voice currents.

17. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having resistances which differwith respect to direct currents and telephone currents, and theresistances of the various lines cliffering from each other, asubscribers set on each line at the outlying station thereof including atransmitter and receiver, a coni mon source of current for said linesand sets at said central office, and a serially connected impedance soproportioned as to render both the direct current supply and thetransmis sion of voice currents over said lines uniform.

18. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having electrical characteristicsdiffering with respect to the transmission of direct currents and voicecurrents and the electrical characteristics of the various linesdiffering from each other, and means adapted to be included in saidlines for producing such uniformity in said electrical characteristicsthat the transmission of line currents from end to end over any pair oflines interconnected through said central station will be uniform.

19. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, each of said lines having electrical characteristicsWhich differ With respect to direct currents and telephone currents andthe electrical characteristics of the various lines differing from eachother, and means to produce uniform transmission of telephone currentsfrom any outlying station to any other outlying station interconnectedtherewith through said central station.

20. In a telephone system, a central station, a plurality of outlyingstations, lines extending from said central station to each of saidoutlying stations, said lines having electrical characteristics whichdiffer with re spect to direct currents and alternating currents and theelectrical characteristics of the various lines differing from eachother, a common source of direct current supply for said lines and saidcentral station, and means for equalizing the supply of said currents toany pair of outlying stations interconnected through said centralstation and for producing uniform transmission of Voice currents betweenany such pair of said stations.

In testimony whereof I have signed my

